Shallow hydrothermal vent systems in Palaeochon Bay, Milos, Aegean Sea were characterised in terms of their flow of fluid, gas, heat and chemicals The chemistry and fluid flow through the sediments around the vents and the activity of micro-organisms inhabiting the ambient seawater were studied using equipment accurately positioned by scientific divers
In situmeasurements confirmed the shallow vent sites to be extreme environments of low pH and high temperature The outflow of gas or hot liquid through the permeable sediment induced a convection cell of pore-water entrainment from deeper in the sediment to the surface Nutrients and dissolved carbon dioxide were transported to the sediment surface across an area of 4m diameter around a single vent and ejected into the water column This outflow was compensated by an inflow of oxygen inch water over an area forming a ring around the outflow region These geochemical conditions were reflected in the colour of the sediment and the type of precipitate or ‘mat’ forming on the sediment surface
Gas and fluid flux measurements made at a number of vent sites indicate the mean flow of fluid discharge in Palaeochon Bay to be in the range 50–80 dm3 h−1Despite the ‘nutrient pump’ effect of the vent system, plankton biomass, photosynthesis and respiration remained low (< 0 1 mg m−3, < 1 mmol O2m −3d−1, 2–4 mmol O2m −3d−1 respectively) The water column was always dominated by respiring rather than photosynthesising organisms No significant difference was found between plankton production at venting and non-venting sites despite a large range of ambient dissolved inorganic carbon concentration. By contrast, small changes in ambient temperature had a profound stimulatory effect on plankton respiration and a lesser effect on plankton photosynthesis
The biogeochemistry of deep-sea hydrothermal vents is currently an area of active research However, there have been relatively few studies of hydrothermal systems on continental shelves The island of Milos is one of the most south-western islands within the Hellenic volcanic arc in the Aegean Sea Submarine venting in these shallow sites is of the gasohydrothermal type where free gas vents together with hydrothermal liquids Extensive areas of submarine gas venting have been recorded for Palaeochon Bay on the south-eastern coast of Milos This gas consisted mainly of carbon dioxide (up to 92 %), with hydrogen, hydrogen sulphide and methane also present (Dando et al., 1995) As part of a European research project (EC Mast I11 Hydrothermal Fluxes and Biological Production in the Aegean) the current study details measurements of the chemical, biological and physical properties of the sediment and water column in the vicinity of shallow water gas and fluid vents in Palaeochon Bay in September 1996 This paper presents an overview of data collected in situby four scientific dive teams and describes how these separate studies can be merged to further understand the flow of gas, liquid and dissolved chemicals through the sediment and into the water column, together with the effect that this venting has on planktonic micro-organisms.
